Ill 



SIGHT— FIELDWORK AND EXPERIMENTS 



Much information about the part that vision plays in the 

 general behaviour of animals may be obtained from observa- 

 tion in the field. This does not lend itself well to an instruc- 

 tional style of writing, and as a great deal of what one wishes 

 to draw attention to depends upon personal experiences it 

 is, perhaps, better to adopt the narrative form in cases where 

 this style will make for greater clarity and interest. 



I have already made reference to quite simple experiments 

 with lower organisms — mostly aquatic — and it is only neces- 

 sary to say that a student can see for himself the way in 

 which light affects many of these animals, some of which are 

 microscopic and a few which are just visible to the naked 

 eye. In either case a microscope is necessary; and though 

 the instrument need not be an elaborate one, it is most 

 satisfactory if one uses a microscope that will take an objec- 

 tive of one-sixth for high-power work, but lower powers 

 down to one inch or even two inches will be invaluable. 



An old-fashioned type of instrument — one with what is 



called a Wenham tube — is really excellent for pond life. This 



type has the advantage over up-to-date models that it will 



rack back sufficiently to allow a low-power objective of two 



inches to be used. Many modern instruments will not allow 



of this. The Wenham tube type is a binocular instrument 



working with a prism which can, at will, be put out of action, 



thus allowing monocular use for higher power work. The 



advantage of using the binocular mechanism is that a degree 



of stereoscopic effect is present when using objectives up to 



half an inch. This is a great help when observing pond life, 



and gives a truer picture of the organism under view. Simple 



accessories such as live-boxes, glass troughs, and slides with 



cavities ground in them are necessary; also setting needles, 



fine forceps and so on. 



so 



